// Based on the ThreadPool code at
// https://github.com/progschj/ThreadPool, modified by Tiziano Peraro.
// The original code comes with the following copyright and license:
//
//
// Copyright (c) 2012 Jakob Progsch, Václav Zeman
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
//    1. The origin of this software must not be misrepresented; you must not
//    claim that you wrote the original software. If you use this software
//    in a product, an acknowledgment in the product documentation would be
//    appreciated but is not required.
//
//    2. Altered source versions must be plainly marked as such, and must not be
//    misrepresented as being the original software.
//
//    3. This notice may not be removed or altered from any source
//    distribution.

#ifndef FFLOW_THREAD_POOL_HH
#define FFLOW_THREAD_POOL_HH

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>

namespace fflow {

  class ThreadPool {
  public:

    ThreadPool() {}
    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args)
      -> std::future<typename std::result_of<F(Args...)>::type>;
    ~ThreadPool();

    void alloc_threads(std::size_t threads);


  private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::function<void()> > tasks;

    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop = false;
  };

  // the constructor just launches some amount of workers
  inline void ThreadPool::alloc_threads(std::size_t threads)
  {
    std::size_t curr_threads = workers.size();
    for (size_t i = curr_threads; i<threads; ++i)
      workers.emplace_back(
                           [this]
                           {
                             for(;;)
                               {
                                 std::function<void()> task;

                                 {
                                   std::unique_lock<std::mutex> lock(this->queue_mutex);
                                   this->condition.wait(lock,
                                                        [this]{ return this->stop || !this->tasks.empty(); });
                                   if(this->stop && this->tasks.empty())
                                     return;
                                   task = std::move(this->tasks.front());
                                   this->tasks.pop();
                                 }

                                 task();
                               }
                           }
                           );
  }

  // add new work item to the pool
  template<class F, class... Args>
  auto ThreadPool::enqueue(F&& f, Args&&... args)
    -> std::future<typename std::result_of<F(Args...)>::type>
  {
    using return_type = typename std::result_of<F(Args...)>::type;

    auto task = std::make_shared< std::packaged_task<return_type()> >(
                                                                      std::bind(std::forward<F>(f), std::forward<Args>(args)...)
                                                                      );

    std::future<return_type> res = task->get_future();
    {
      std::unique_lock<std::mutex> lock(queue_mutex);

      // don't allow enqueueing after stopping the pool
      if (stop)
        std::terminate();

      tasks.emplace([task](){ (*task)(); });
    }
    condition.notify_one();
    return res;
  }

  // the destructor joins all threads
  inline ThreadPool::~ThreadPool()
  {
    {
      std::unique_lock<std::mutex> lock(queue_mutex);
      stop = true;
    }
    condition.notify_all();
    for(std::thread &worker: workers)
      worker.join();
  }

} // namespace fflow


#endif // FFLOW_THREAD_POOL_HH
